The present invention relates to a method (hereinafter referred to as of the kind described) for introducing solid, liquid or gaseous fuel into a burning zone of a kiln, such as a rotary kiln for manufacturing cement clinker or similar products, by which method fuel is conducted through a duct or ducts and primary air is conducted through at least two annular ducts arranged substantially concentrically with, and around, the fuel duct(s) , the portion of air in one of the air ducts flowing substantially axially, whereas the portion of air in the second of the air ducts comprises air which has a rotary component about the centre axis of the burner, and the amount of primary air in the two portions being independently controlled. The invention also relates to a burner for carrying out the method according to the invention.
Burners for this purpose are well-known. Originally, they merely consisted of one single pipe through which a mixture of pulverized coal meal and air was injected into the burning zone of the kiln. Over time, design improvements of the burners were implemented, with the incorporation of features such as additional ducts for introducing other types of liquid or gaseous fuel. Furthermore, most modern burners comprise one or several separate ducts for injection of air, so that only a small amount of the primary air is injected together with the pulverized coal. By imparting a rotary motion to some of the injected air it has, to a larger extent, been possible to control the flame shape in the kiln.
An example of a burner of the above mentioned kind is described in EP-B-0421903. This known burner comprises one or several ducts for introduction of fuel, being surrounded by two annular ducts for injecting primary air. In the annular nozzle opening of the innermost of these air ducts are provided oblique blades which impart a rotary motion to the air. In the outermost duct the air is conducted and injected in a substantially axial direction. An adjustment of the nozzle area of both ducts can be made by adjusting the ducts axially relative to one another, and the portions of primary air in the two ducts can also be independently controlled. Variability of flame shape is, therefore, possible with this burner given the possibility of adjusting the flow rate and velocity of the primary air as well as the amount of primary air which is subjected to rotation. However, the drawback of this burner is that the primary air is injected through two separate annular nozzles, resulting in a relatively high pressure loss and a less effective mixing of the primary air with the fuel in the burning zone.
A second example of a burner of the above mentioned kind is described in EP-A-0650012. This known burner also comprises one or several ducts for the introduction of fuel, surrounded by one single primary air duct which discharges into an annular nozzle. Immediately ahead of the nozzle the air is directed through a number of flexible tubes which, by means of a mechanism, can be bent sideways, thereby causing the air to rotate. The rotation of the air, and hence the flame shape, can thus be varied by changing the angle of bending of the tubes, and by changing the amount of primary air. The advantage of feeding all of the primary air through only one nozzle is that it will reduce the loss of pressure and ensure a more effective mixing of air and fuel, and hence a more steady flame. However, the drawback of this type of burner is that the system with the flexible tubes requires a relatively complex regulating mechanism which also appears to be vulnerable in its intended operating environment.